Method of preparing latex for coating paper

ABSTRACT

The present invention relates to styrene-butadienie latex, specifically a method of preparing latex for coating paper which shows excellent adhesion while maintaining printing property and comprised in a composition for coating paper. To achieve the object of the present invention, it provides a method of preparing styrene-butadiene latex comprising the step of emulsion polymerization by adding 0.1 to 10 parts by weight of chain transfer agent comprising; i) monofunctional thiol compound, and ii) polyfunctional thiol compound having at least two thiol groups based on 100 parts by weight of styrene-butadiene monomer composition. The latex prepared by the present method can easily control molecular weight and gel content, so that polymerization stability and mechanical stability are improved and thus, it is possible to print paper with high speed and to improve adhesion.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/KR01/02221 which has an Internationalfiling date of Dec. 20, 2001, which designated the United States ofAmerica.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to styrene-butadiene latex, specifically amethod of preparing latex for coating paper which shows excellentadhesion while maintaining printing property and comprised in acomposition for coating paper.

(b) Description of the Related Art

In preparing paper for coating, recently the companies reduce the costprice through increase of coating speed, addition of a functionalinorganic pigment and reduction of amount of binders. Specifically, inlatex comprised in coating composition, mechanical stability andpolymerization stability by small-particlization are very importantfactors, and adhesion becomes a very important one of printingproperties.

A conventional coating method of paper with coating compositioncomprises transferring coating solution from an applicator-roll tosurface of paper and removing excess coating solution by a blade or anair-knife. In this case, since the coating solution is subjected to highpressure, high shear fluidity and mechanical stability that is differentfrom polymerization stability of latex are very important. Thus, ascoating speed increases, such mechanical stability and high shearfluidity will be important and they will be important factors todetermine the productivity of coating paper.

If the stability of latex is destructed under the high pressure, thecoating solution around the applicator-roll is gummed up, which causesquality deterioration of paper as well as remarkable reduction ofcontinuous product of paper.

In addition, in preparing latex for coating paper, since latex is usedas a binder, a lot of scales are produced in a stirrer or inner side ofa reactor. As latex is made to small particles, the coagulants increase.They act as impurities in the following process, which results inunevenness of material properties in the continuing process anddecreases productivity because much time and effort will be needed toremove them.

Unlike the above bulk coagulant in the reactor, fine coagulum exists inthe suspension. It tends to increase the amount as latex is made tosmall particles. The coagulum is screened after complete ofpolymerization and during preparing coating solution by screening. Ifthere are many impurities, it is difficult to control products becauseit needs much time and cost. Also, a severe problem such as streakoccurs on the surface of paper so that it deteriorates the quality ofpaper.

As latex is made to small particles and concentration of solid ratioincreases gradually, it is important for latex to have polymerizationstability. The polymerization stability relates to improvement ofproductivity and high-shear fluidity as well as increase ofproductivity. This also lessens unevenness of material properties in thecontinuous producing so that it is helpful to improve qualityrelatively.

Due to recent high-speed coating, it is required to have stability oflatex and due to high speed printing tendency, it becomes important tohave adhesion.

The above paper coating composition comprises a pigment, adhesive andother additives. The synthetic latex such as styrene-butadiene latex ismainly used as the adhesive. The adhesive significantly affects thequality of printing paper, specifically adhesion.

The important printing properties include adhesion (dry pickresistance), drying speed of ink, gloss, water resistance, and wet inkreceptivity.

Due to high cost of latex of the coating composition and the tendency ofhigh-speed printing, the importance of adhesion becomes high because thecompanies require reducing the amount of latex. That is, the coatingpaper should resist strong mechanical strength against surface ofcoating paper of pigments with as less amount of latex as possible sothat it does not occur exfoliation from coating layer and falling off ofpigment, thus, the coating paper can have clear printing appearance.

In addition, factors of styrene-butadiene latex affecting to adhesion ofcoating paper are glass transition temperature, particle size,composition of monomers and so on. Generally, adhesion shows high valueat determined s gel content and the optimized gel content value dependson the composition of monomers.

Another important factor is a drying speed of ink. In the case of multicolor printing, printing is done by four colors of blue, black, red, andyellow. As printing speed increases, time term to print the next colorbecomes short, lo thus it is required faster drying speed of ink. If inkis not well dried and goes to the next step, print mottle or post-smearcan occur. The relationship of gel content and drying speed of inkaffects film-formability according to gel content as well as swellingindex showing amount of solvent which latex particle can contain.

Gloss is an important factor to increase commercial value and quality ofprinting paper. Gloss includes paper gloss of coating paper and printinggloss after printing. As the value of both of them increases, paper hasgood appearance.

In order to increase paper gloss, generally particle size of latexshould increase or content of latex should decrease. However, thosemethods have a demerit of lowering adhesion. In order to increaseprinting gloss, air permeability should decrease so that it is requiredto hold solvent on the surface until having stable arrangement afterprinting. Thus, it needs to decrease drying speed of ink.

In addition, water resistance is also important printing property ofoffset printing. In the offset printing, water is used in the printingprocess. If water resistance(wet pick resistance)decreases, pigments canbe exfoliated by applied strong physical strength during printing.

As like adhesion, water resistance and gel content have a relationship.That is, water resistance is the strongest at determined gel content.However, gel content that shows the strongest water resistance and gelcontent that shows the highest adhesion do not coincide. There is atendency that water resistance shows the maximum point at lower gelcontent and adhesion shows the maximum point at higher gel content.

Another printing property required in the offset printing is wet inkreceptivity. As described in the above, since water is used in theoffset printing, if coating paper does not effectively absorb water whenprinting, ink that does not have compatibility with water will notadhere to coating paper, which results in low degree of printing.Generally, wet ink receptivity and water resistance are oppositeproperties so that it is difficult to increase both of them.

As above, it is very difficult to prepare latex that can provide coatingpaper with excellent printing properties and coating and printingcircumstances become stricter.

SUMMARY OF THE INVENTION

Considering the above problems of prior art, the present inventionprovides a method of preparing latex that has remarkably improvedpolymerization stability and mechanical stability by controllingmolecular weight, gel content and structure of latex.

It is another object of the present invention to provide a method ofimproving productivity of paper by increasing concentration of solidratio used in producing paper, which by improving quality of latex bysmall-particlization.

In order to achieve the above objects, the present invention provides amethod of preparing styrene-butadiene latex comprising the step ofemulsion polymerization by adding 0.1 to 10 parts by weight of chaintransfer agent comprising;

-   -   i) monofunctional thiol compound, and    -   ii) polyfunctional thiol compound having at least two thiol        groups based on 100 parts by weight of styrene-butadiene monomer        composition.

More specifically, the present invention provides a method of preparingstyrene-butadiene latex comprising:

a) a step of preparing seed latex by emulsion polymerizing styrene andbutadiene monomer composition and chain transfer agent;

b) a step of preparing first-shell covered latex by adding coveringcomposition comprising styrene and butadiene monomer composition andchain transfer agent to said seed latex and emulsion polymerizing them;and

c) a step of preparing second-shell covered latex by adding coveringcomposition comprising styrene and butadiene monomer composition andchain transfer agent to said first-shell covered latex and emulsionpolymerizing them;

In addition, the present invention provides styrene-butadiene latexprepared by the above method.

Also, the present invention provides a paper coating compositioncomprising latex prepared by the above method.

In addition, the present invention provides paper prepared by coveringthe above paper coating composition.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be explained in detail.

As a result of extensive researching latex for coating paper, thepresent inventors found that styrene-butadiene latex with improvedpolymerization stability and mechanical stability and excellent printingproperty can be prepared by controlling gel content and structure oflatex. Thus, the present invention was accomplished based on this.

In preparing latex, only monofunctional thiol compound such as alkylmercaptan has been conventionally used as a chain transfer agent. Thepresent invention, however, provides a method of preparingstyrene-butadiene latex that can control molecular weight, gel contentand structure of latex by using polyfunctional thiol compound havingtwo, three or four thiol groups in a molecule adding to monofunctionalthiol. The latex prepared by this method has better polymerizationstability and mechanical stability as well as better adhesion than theprior latex so that coating paper prepared by using the latex has goodprinting properties and can be printed with high speed.

To achieve the above object, the present invention comprises a step ofemulsion polymerization styrene-butadiene monomer composition by addinga determined amount of chain transfer agent comprising monofunctionalthiol compound and polyfunctional thiol compound.

In this case, monofunctional thiol compound of chain transfer agent isessentially used in all steps and in preparing seed latex,monofunctional thiol compound can be used alone. The abovepolyfunctional thiol compounds control molecular weight, gel content andstructure of latex by mixing with monofunctional thiol compound in thesecond step and third step in which covering the first shell and thesecond shell respectively.

Preparing latex in the present invention comprises two steps or multisteps. Conventionally, latex is prepared by emulsion polymerization bycontinuously covering one to three shells to seed latex which wasprepared previously. The other reaction conditions such aspolymerization initiator, emulsifier, electrolyte and so on are the sameto the known art. That is, preparation of latex comprises a) a step ofpreparing seed latex; b) a step of preparing first-shell covered latex;and c) a step of preparing second-shell covered latex.

The above step a), that is, a step of preparing seed latex comprisesadding styrene and butadiene monomer composition, chain transfer agentand polymerization initiator and polymerizing them.

The above step b), that is, a step of preparing first-shell coveredlatex comprises adding styrene and butadiene monomer composition, chaintransfer agent and polymerization initiator of which amounts weremodified and polymerizing them.

Also, the above step c), that is, a step of preparing second-shellcovered latex comprises adding styrene and butadiene monomercomposition, chain transfer agent and polymerization initiator of whichamounts were modified and polymerizing them so that latex controlledmolecular weight and gel content is prepared.

The amount of styrene and butadiene monomer compositions used in theabove steps are 20–55 parts by weight of 1,3-butadiene, 45–80 parts byweight of styrene, and 1–15 parts by weight of ethylene unsaturated acidmonomer based on 100 parts by weight of all monomers.

1,3-butadiene provides flexibility for copolymer. When the content isless than 20 parts by weight, the copolymer becomes harden and when thecontent is more than 55 parts by weight, water resistance decreases.

Styrene provides suitable hardness and water resistance. When thecontent is less than 45 parts by weight, the effect is not enough andwhen the content is more than 80 parts by weight, adhesion andfilm-formability may decrease.

Ethylene unsaturated acid monomer is properly used to improve adhesionof polymers and to modify stability of latex particles. It is preferably2 to 9 parts by weight. When the content is less than 1 part by weight,the above effect is not obtained and when the content is more than 15parts by weight, there seems to occur problem such as polymerizationstability. Preferable examples of the ethylene unsaturated acid monomerare at least one selected from the group consisting of unsaturatedcarboxylic acid of methacrylic acid, acrylic acid, itaconic acid,chrotonic acid, fumaric acid or maleic acid; and unsaturatedpolycarboxylic alkyl ester which having at least one carboxylic group ina molecule such as itaconic acid monoethyl ester, fumaric acid monobutylester, or maleic acid monobutyl ester.

The amount of styrene and butadiene monomer compositions furthercomprises 0.1–20 parts by weight of cyanide vinyl monomer, and 0.1–30parts by weight of copolymerizable vinyl monomer based on 100 parts byweight of total monomers.

The cyanide vinyl monomer is effective to improve printing gloss and theamount is preferably 3 to 8 parts by weight. The specific example isacrylonitrile or methacrylonitrile.

When synthesizing copolymer latex in the present invention, ifnecessary, vinylic monomer copolymerizable with the above monomers canbe used. The example of these monomers is at least one selected from thegroup consisting of unsaturated carboxylic acid alkyl ester ofmethylacrylate, methylmethacrylate, ethylacrylate, ethylmethacrylate,butylacrylate, or butylmethacrylate; unsaturated carboxylic acidhydroxyalkyl ester of β-hydroxyethyl acrylate, β-hydroxypropyl acrylate,or β-hydroxyethyl methacrylate; unsaturated carboxylic acid amide ofacrylamide, methacrylamide, itaconamide, or maleic acid monoamide andderivatives thereof; and aromatic vinyl monomer such as α-methylstyrene,vinyl toluene, or p-methylstyrene. The above unsaturated carboxylic acidalkyl ester provides suitable hardness for copolymer and improvesfilm-formability. The content is preferably 3–15 parts by weight. Whenthe content is more than 30 parts by weight, it has bad influence towater resistance. The above unsaturated carboxylic acid amide andderivatives thereof are useful to improve chemical stability, mechanicalstability and water resistance of copolymer latex. The content ispreferably less than 10 parts by weight.

As described in the above, a chain transfer agent used in the presentinvention is a mixture of monofunctional thiol compound such as alkylmercaptan and polyfunctional thiol compound having at least two thiolgroups in a molecule in a ratio of 1:0.1 to 1:10, which can controlmolecular weight of copolymer, gel content and gel structure. The chaintransfer agent comprising the above compounds is used in a ratio of 0.1to 10 parts by weight based on 100 parts by weight of styrene andbutadiene monomer composition and it is preferably 0.1 to 2.0 parts byweight. When less than 0.1 parts by weight, it is difficult to show theeffect and when more than 10 parts by weight, it has bad effect toreaction speed and reaction stability. The above monofunctional thiolcompound can be n-dodecyl mercaptan or t-dodecyl mercaptan. Also, thespecific example of polyfunctional thiol chain transfer agent is atleast one selected from the group consisting of 1,5-pentanedithiol,1,6-hexanedithiol, 2-ethylhexyl-3-mercaptopropionate, butyl3-mercaptopropionate, dodecyl 3-mercaptopropionate, ethyl2-mercaptopropionate, ethyl 3-mercaptopropionate, methyl3-mercaptopropionate, pentaerythritol tetrakis (3-mercaptopropionate),2-ethylhexyl mercaptoacetate, ethyl 2-mercaptoacetate,2-hydroxymethyl-2-methyl-1,3-propanethiol and pentaerythritol tetrakis(2-mercaptoacetate).

The latex prepared by this method has glass transition temperature of−20 to 25° C., preferably −5 to 15° C. Also, particle size of latex is80 to 200 nm, preferably 100 to 150 nm. If particle size is less than100 nm, low-shear fluidity increases, and paper gloss, ink-drying speedand wet ink receptivity decrease. On the contrary, if particle size ismore than 150 nm, high-shear fluidity increases, and printing gloss,adhesion and water resistance decrease. In addition, gel content oflatex is 40–95%, preferably 60–80%.

As above, the properties of latex prepared by the above method can becontrolled so that when preparing coating paper, the paper comprisingthe coating composition has improved polymerization stability,mechanical stability and adhesion.

The present invention is further explained in more detail with referenceto the following EXAMPLES. These examples, however, should not in anysense be interpreted as limiting the scope of the present invention.

The particle sizes and gel contents of Examples 1 to 6 and ComparisonExamples 1 to 3 are measured as follows.

1) Particle size of latex: Laser Scattering Analyzer (Nicomp) was used.

2) Gel content: Latex finished polymerization was set to pH 7–8 anddried at normal temperature for more than 24 hours. When film is formedenough, it was cut to determined size, placed into 80-mesh net, anddissolved in excess of tetrahydrofuran for 14 hours. The content ofnon-dissolved is shown in percent.

EXAMPLES Example 1

According to the following three steps, latex was prepared and theingredients used in each step were shown in the below-table 1.

First step: 10L-pressure reactor equipped with a stirrer, a thermometer,a cooler, and an inlet of nitrogen gas and equipped so as tocontinuously provide monomers, emulsifiers and a polymerizationinitiator was purged with nitrogen and then was added with components offirst step of table 1 and was heated to 65° C. Thereto, 1 part by weightof potassium persulfate as a polymerization initiator was added andstirred for 300 minutes to complete polymerization of seed latex. Theobtained seed has an average particle size of 70 nm, 98% of conversionrate, and 86% of gel content.

Second step: In order to cover the first shell into the above seed latexobtained from the first step, 28 parts by weight of seed latex was addedand was heated to 75° C. The components of second step of table 1 werecontinuously added for 150 minutes to polymerize them. After adding allcomponents, they were further stirred for 60 min to completepolymerization. The obtained latex with first shell has an averageparticle size of 105 nm, 90% of conversion rate, and 62% of gel content.

Third step: In order to cover the second shell into the above latexobtained from the second step, components of third step of table 1 werecontinuously added for 60 minutes while maintaining 75° C. of reactorwhich was filled with 507 parts by weight of latex obtained in thesecond step. After adding all components, they were further stirred for200 min to complete polymerization. The obtained final latex with secondshell has an average particle size of 123 nm, 98% of conversion rate,and 75% of gel content.

TABLE 1 Example 1 Components (Parts by weight) First step Second stepThird step Butadiene 33 39 40 Styrene 42 43 42 Methylmethacrylate 12 8 4Acrylonitrile 8 4 9 Itaconic acid 5 2 2 Acrylic acid — 4 3 Sodiumdodecyl dibenzene 6 0.9 0.3 sulfonic acid Chain transfer agent t-dodecylmercaptan 0.15 1.0 0.8 Pentaerythritol tetrakis — 1.2 1.4 Sodiumbicarbonate 0.5 0.4 0.4 Ion exchange water 420 66 79 Potassiumpersulfate 1 2.5 2.0

Examples 2 to 4

All reaction conditions and processes are the same to Example 1 exceptthat kind and amount of chain transfer agent are varied as table 2.

TABLE 2 Components (Parts by weight) First step Second step Third stepExample 2 t-dodecyl mercaptan 0.15 0.7 1.1 3-mercaptopropionate 0 1.51.0 Example 3 t-dodecyl mercaptan 0.15 1.0 0.8 2-mercapto acetate 0 1.21.4 Example 4 t-dodecyl mercaptan 0.15 0.7 1.1 2-mercapto acetate 0 1.51.0

Example 5

First step: same to Example 1.

Second step: In order to cover the first shell into the above seed latexobtained from the first step, 28 parts by weight of seed latex was addedinto the reactor and was heated to 75° C. The components of second stepof table 3 were continuously added for 150 minutes to polymerize them.After adding all components, they were further stirred for 60 min tocomplete polymerization. The obtained latex with first shell has anaverage particle size of 107 nm, 90% of conversion rate, and 71% of gelcontent.

Third step: In order to cover the second shell into the above latexobtained from the second step, components of third step of table 3 werecontinuously added for 60 minutes while maintaining 75° C. of reactorwhich was filled with 507 parts by weight of latex obtained in thesecond step. After adding all components, they were further stirred for200 min to complete polymerization. The obtained final latex with secondshell has an average particle size of 125 nm, 98% of conversion rate,and 79% of gel content.

TABLE 3 Example 5 Components (Parts by weight) First step Second stepThird step Butadiene 33 40 42 Styrene 42 42 38 Methylmethacrylate 12 6 6Acrylonitrile 8 6 9 Itaconic acid 5 3 2 Acrylic acid — 3 3 Sodiumdodecyl dibenzene sulfonic 6 0.8 0.4 acid Chain transfer agent t-dodecylmercaptan 0.15 1.0 0.8 Pentaerythritol tetrakis — 1.2 1.4 Sodiumbicarbonate 0.5 0.4 0.4 Ion exchange water 420 66 79 Potassiumpersulfate 1 2.5 2.0

Example 6

All reaction conditions and processes are the same to Example 5 exceptthat kind and amount of chain transfer agent are varied as table 4.

TABLE 4 Components (Parts by weight) First step Second step Third stepExample 6 t-dodecyl mercaptan 0.15 0.7 1.1 3-mercaptopropionate 0 1.51.0

Comparative Examples 1 to 2

All reaction conditions and processes are the same to Example 1 exceptthat only monofunctional compounds as a chain transfer agent are used toprepare latex in each step and the content thereof is shown in table 5.

TABLE 5 Components (Parts by weight) First step Second step Third stepCom. Ex. 1 t-dodecyl mercaptan 0.15 0.8 1.4 Com. Ex. 2 t-dodecylmercaptan 0.15 1.4 0.8

Comparative Example 3

All reaction conditions and procedures are the same to Example 5 exceptthat only monofunctional thiol compounds as a chain transfer agent areused to prepare latex in each step and the content thereof is shown intable 6.

TABLE 6 Components (Parts by weight) First step Second step Third stepCom. Ex. 3 t-dodecyl mercaptan 0.15 0.8 1.4

Experimental Example 1

(Properties Test of Latex)

In order to measure polymerization stability of latex prepared in theabove Examples 1 to 6 and Comparative Examples 1 to 3, they were passedthrough 150-, 200-, 325-mesh net respectively and the amount of scaleswere measured by ppm based on the solid ratio and shown in table 7.

TABLE 7 Com. Com. Com. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 1 Ex. 2Ex. 3 150- <20 <10 <30 <20 <50 <30 <200 <100 <200 mesh 200- <20 <10 <30<20 <50 <30 <200 <100 <200 mesh 325- <50 <20 <60 <50 <80 <80 <200 <100<400 mesh

In addition, in order to compare polymerization stability of Examples 1to 6 and Comparative Examples 1 to 3, inner scale was weighed aftercompletion of polymerization and shown in table 8.

TABLE 8 Com. Com. Com. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 1 Ex. 2Ex. 3 Weight 40 15 40 25 65 55 >500 >500 >800 of inner scale (g)

Example 7

(Preparation of Coating Composition)

In order to compare and evaluate latex of Examples 1 to 6 andComparative Examples 1 to 3, paper-coating solutions as in table 9 wereprepared. The distilled water was added as to have 67.3% of solid ratio.

TABLE 9 paper-coating solutions Content (parts by weight) First gradeclay 57 Calcium carbonate 43 Styrene-butadiene latex 12 Oxidized starch1.3

Example 8

(Preparing Coating Paper)

Coating paper was obtained by coating paper-coating solutions preparedin Example 7 according to the following conditions.

Coating: Rod Coating, No.6

Drying: Oven 105° C., 30 seconds

Calendar: Super calendar, 80° C., 100 kg/cm, 4 m/min, 2 times pass

Original Paper: Commercially available paper (average weight 72 gsm)

The properties of these were measured as following method and shown intable 10.

1) Adhesion:

After printing several times in the RI printer, the degree of tear wasmeasured by naked eyes and evaluated by 5-score method. As the scoreincreases, adhesion becomes better. After Ink of tack value 12, 14, and16 was used and measured, an average was obtained.

2) Water Resistance

After adding wet pick water by using molten-roll in the RI printer andprinting, the degree of tear was measured by the same method as that ofadhesion. The value was measured by using ink of tack value 14 afterprinting one time.

3) Drying Speed of Ink

After printing in the RI printer, the degree of smear was measured by5-score method. As the score increases, the drying speed of ink is high.

4) Wet Ink Receptivity

After adding water in the RI printer and printing, the degree oftransfer of ink was measured by 5-score method. Ink of low tack valuewas used in order not to occur tear. As the score increase, the wet inkreceptivity is high.

5) Paper Gloss

Average value was measured after measuring various parts of coatingpaper using Optical Gloss Meter (HUNTER type, 75° gloss)

6) Printing Gloss

This is measured in the same method as in the paper gloss after 24 hoursafter printing in RI printer.

TABLE 10 Ex. Com. Com. Com. Properties Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 6Ex. 1 Ex. 2 Ex.3 Adhesion 4.4 4.5 4.3 4.3 4.5 4.6 3.9 4.0 4.0 Water 4.14.2 4.2 4.1 4.3 4.3 4.0 4.1 4.1 resistance Drying 4.2 4.1 4.1 4.0 3.93.9 4.1 4.0 3.9 speed of ink Wet ink 4.2 4.2 4.3 4.3 4.2 4.2 4.2 4.3 4.2receptivity Paper 71 72 71 71 69 70 70 71 69 gloss (%) Printing 81 81 8081 78 78 80 81 78 gloss (%)

The styrene-butadiene latex prepared by the present invention can easilycontrol molecular weight, gel content, and molecular weight by usingpolyfunctional thiol compound as a chain transfer agent so that it ispossible to print paper with high speed and to improve adhesion whilemaintaining various printing properties. In addition, it is possible toreduce the weight of latex in preparing coating composition.

1. A method of preparing styrene-butadiene latex comprising the step ofemulsion polymerization by adding 0.1 to 10 parts by weight of chaintransfer agent comprising: i) monofunctional thiol compound, and ii)polyfunctional thiol compound having at least two thiol groups based on100 parts by weight of styrene-butadiene monomer composition.
 2. Themethod of claim 1, wherein said polymerization comprises: a) a step ofpreparing seed latex by emulsion polymerizing styrene and butadienemonomer composition and chain transfer agent; b) a step of preparingfirst-shell covered latex by adding covering composition comprisingstyrene and butadiene monomer composition and chain transfer agent tosaid seed latex and emulsion polymerizing them; and c) a step ofpreparing second-shell covered latex by adding covering compositioncomprising styrene and butadiene monomer composition and chain transferagent to said first-shell covered latex and emulsion polymerizing them.3. The method of claim 1, wherein said monofunctional thiol compound isn-dodecyl mercaptan or t-dodecyl mercaptan.
 4. The method of claim 1,wherein said polyfunctional thiol compound is at least one selected fromthe group consisting of 1,5-pentanedithiol, 1,6-hexanedithiol,2-ethylhexyl-3-mercaptopropionate, butyl 3-mercaptopropionate, dodecyl3-mercaptopropionate, ethyl 2-mercaptopropionate, ethyl3-mercaptopropionate, methyl 3-mercaptopropionate, pentaerythritoltetrakis (3-mercaptopropionate), 2-ethylhexyl mercaptoacetate, ethyl2-mercaptoacetate, 2-hydroxymethyl-2-methyl-1,3-propanethiol andpentaerythritol tetrakis (2-mercaptoacetate).
 5. The method of claim 1,wherein weight ratio of said monofunctional compound to saidpolyfunctional thiol compound which having at least two thiol groups is1:0.1 to 1:10.
 6. The method of claim 1, wherein said styrene andbutadiene monomer composition comprises 20 to 55 parts by weight of1,3-butadiene, 45 to 80 parts by weight of styrene, and 1 to 15 parts byweight of ethylene-unsaturated acid monomer based on 100 parts by weightof total monomer.
 7. The method of claim 1, wherein said styrene andbutadiene monomer composition further comprises 0.1 to 20 parts byweight of cyanide vinyl monomer and 0.1 to 30 parts by weight of vinylmonomer copolymerizable with the cyanide vinyl monomer based on 100parts by weight of total monomer.
 8. The method of claim 7, wherein saidethylene-unsaturated acid monomer is at least one selected from thegroup consisting of unsaturated carboxylic acid of methacrylic acid,acrylic acid, itaconic acid, chrotonic acid, fumaric acid or maleicacid; and unsaturated polycarboxylic alkyl ester which having carboxylicgroup of itaconic acid monoethyl ester, fumaric acid monobutyl ester, ormaleic acid monobutyl ester.
 9. The method of claim 7, wherein saidcyanide vinyl monomer is acrylonitrile or methacrylonitrile.
 10. Themethod of claim 7, wherein said copolymerizable vinylic monomer is atleast one selected from the group consisting of unsaturated carboxylicacid alkyl ester of methylacrylate, methylmethacrylate, ethylacrylate,ethylmethacrylate, butylacrylate, or butylmethacrylate; unsaturatedcarboxylic acid hydroxyalkyl ester of β-hydroxyethyl acrylate,β-hydroxypropyl acrylate, or β-hydroxyethyl methacrylate; unsaturatedcarboxylic acid amide of acrylamide, methacrylamide, itaconamide, ormaleic acid monoamide and derivatives thereof; and aromatic vinylmonomer such as α-methylstyrene, vinyl toluene, or p-methylstyrene. 11.A styrene-butadiene latex prepared by the method of claim
 1. 12. Thelatex of claim 11, wherein said latex has gel content of 40–90%, glasstransition temperature of −20 to 25° C., and average particle size of 80to 200 nm.
 13. A paper coating composition comprising thestyrene-butadiene latex of claim
 11. 14. A coating paper prepared bycovering a paper with the paper coating composition of claim 13.